This invention relates to a furnace combustion system.
A furnace may provide heat by burning fuel gas in a combustible mixture with oxidant gas. Such a furnace includes one or more burners, and has a reactant supply system with lines and valves that direct streams of the fuel and oxidant gases to the burners. In some furnaces the fuel and oxidant gases are provided to the burners in separate streams that form combustible mixtures within the burners. In other furnaces the fuel and oxidant gases are provided to the burners in a combustible mixture known as premix. In each case, the furnace has a control system that controls the reactant supply system so that combustion of the fuel will occur in a manner appropriate for the heating process to be performed by the furnace. The control system includes instruments such as temperature sensors, igniters, and flame detectors, and controls the valves in the reactant supply system with reference to combustion parameters indicated by those instruments.
Parts of a known furnace apparatus 10 are shown schematically in FIGS. 1 and 2. These include a burner assembly 12 mounted in a furnace wall structure 14. The wall structure 14 defines a combustion chamber 15, which may be referred to as a process chamber, with a flue gas outlet 16. A reactant supply system 20 provides reactants to the burner assembly 12 under the direction of a control system 22. This enables the burner assembly 12 to provide a controlled amount of heat for a heating process to be carried out in the combustion chamber 15.
The reactant supply system 20 includes fuel and oxidant supply lines 24 and 26 that convey fuel and oxidant from respective sources 28 and 30 to the burner assembly 12. The oxidant is typically the oxygen in a stream of air, and the fuel is typically natural gas. A motorized oxidant valve 32 controls the stream of air in the oxidant supply line 26. A pair of motorized fuel valves 34 and 36 control the stream of fuel in the fuel supply line 24. The oxidant valve 32 and the first fuel valve 34 are modulating valves. The second fuel valve 36 is a shut-off valve. Each valve motor is operated by the control system 22 to open, shift, and close the respective valve 32, 34 or 36 as directed by the control system 22.
The burner assembly 10 has two major portions 40 and 42 that are shown schematically in FIG. 1. The first portion 40 is a burner tile with an internal reaction zone 43. A generally cylindrical inner surface 44 of the burner tile 40 is centered on an axis 45, and defines the peripheral boundaries of the reaction zone 43. An outer end surface 46 of the burner tile 40 defines an outlet 47 that communicates the reaction zone 43 with the combustion chamber 15. The outlet 47 is circular and also is centered on the axis 45. An end portion 48 of the inner surface 44 is tapered radially inward so that the outlet 47 is constricted relative to the generally cylindrical configuration of the reaction zone 43.
The second major portion 42 of the burner assembly 10 comprises a burner with a fuel inlet 50 and an oxidant inlet 52. Passages within the burner 42 receive the streams of fuel and oxidant from the inlets 50 and 52 to form a combustible mixture, and the burner 42 is oriented to fire a flame into the reaction zone 43 in a direction extending into the combustion chamber 15 through the outlet 47.
The control system 22 includes a controller 60. The control system 22 further includes a temperature sensor 62 which is operative in the combustion chamber 15. Additional parts of the control system 22 that are shown schematically in FIG. 1 include an igniter 64 and a flame detector 66. These are conventional devices that are operatively mounted in the burner assembly 10 in a known manner.
Other parts of the burner assembly 12, the furnace wall structure 14, the reactant supply system 20 and the control system 22 are known to a person of ordinary skill in the art. Those parts, such as a flame stabilizer within the burner assembly 12, are omitted from the drawings for clarity of the schematic illustrations.
As indicated in FIG. 1, the controller 60 includes flame supervisory controls in the form of hardware and/or software 70 for operation of the furnace apparatus 10 with flame supervision, and further has a flame supervision bypass function, including hardware and/or software 72 for bypassing flame supervision. As the controller 60 carries out those instructions, it first actuates the valves 32, 34 and 36 to provide streams of fuel and oxidant along the supply lines 24 and 26, and actuates the igniter 64 to initiate combustion of those reactants in a flame that projects from the burner 42.
As the streams of fuel and oxidant continue to flow to the burner 42, the controller 60 provides flame supervision in accordance with the corresponding flame supervisory controls 70. Flame supervision is one of several supervisory functions the controller 60 performs by monitoring sensors that can indicate system malfunctions. If a malfunction occurs, the controller 60 can respond by closing the shut-off valve 36 as a safety precaution. However, if a malfunction does not present an unsafe condition, the controller 60 can bypass the supervisory function for the corresponding sensor, and can allow combustion to continue. The controller 60 thus monitors the flame detector 66 in readiness to close the shut-off valve 36 if the flame detector 66 indicates the absence of a flame. This would occur if the flame were inadvertently extinguished by a system malfunction. However, the controller 60 monitors the temperature sensor 62 also, and is operative to compare the sensed combustion chamber temperature to a predetermined auto-ignition temperature of the fuel supplied to the burner 42. If the sensed combustion chamber temperature is not less than the auto-ignition temperature, and if the flame supervision bypass function 72 is enabled, the flame supervisory controls 70 are bypassed. With the flame supervisory controls 70 bypassed, indication by the flame detector 66 of the absence of a flame projecting from the burner 42 will not result in the controller 60 closing the shut-off valve 36. Rather, the controller 60 then holds the shut-off valve 36 open so that the fuel can continue to flow through the burner 42 and onward through the reaction zone 43 to enter the combustion chamber 15 through the outlet 47. This results in diffuse combustion of the fuel upon auto-ignition in the combustion chamber 15 in the absence of a flame at the burner 42, as shown in FIG. 2.
The invention provides a method and apparatus for inducing a diffuse combustion mode of operation in a furnace, whereas the diffuse combustion described above is an unpredictable default condition that occurs when the flame is inadvertently extinguished.
The method includes the step of extinguishing a flame fired from a burner in a reaction zone upon determining that a sensed combustion chamber temperature is not below a predetermined auto-ignition temperature of a fuel. A subsequent step is performed before the sensed combustion chamber temperature drops below the auto-ignition temperature. The subsequent step provides a flow of the fuel through the burner and into the combustion chamber through a reaction zone outlet. This initiates diffuse combustion of the fuel by auto-ignition in the combustion chamber in the absence of a flame fired from the burner in the reaction zone.
The flame is can be extinguished in any one or more of a number of different ways including, for example, interrupting a flow of fuel to the burner, applying a gas pulse, or providing reactants to the burner in a fuel/oxidant ratio that does not sustain the flame. A delay may be imposed to ensure that the flame is fully extinguished before the flow of fuel is provided through the burner for auto-ignition in the combustion chamber. The combustion chamber temperature can be monitored with reference to the auto-ignition temperature of the fuel during the delay to ensure that the flow of fuel is provided before the combustion chamber temperature drops below the auto-ignition temperature. The absence of the flame can be confirmed by the use of a flame detector, either with or without imposing this delay.
The invention further provides a method of modifying a prior art furnace apparatus by rendering the prior art control system operative to perform as summarized above. It follows that the invention further includes the modified apparatus as well as a newly constructed apparatus.